With the development of miniaturization of electronic parts, entertainment electronics and computers, magnetic information stores are increasingly required to have higher storage densities. This can only be achieved by increasing the fineness of sub-division and the coercive force of the magnetic powders used for producing the magnetic recording carriers.
According to Ullmanns Encyklopadie der technischen Chemie, Volume 16, 4th Edition, 1979, pages 361 to 367, needle shaped particles of .gamma.-Fe.sub.2 O.sub.3, cobalt-modified .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-modified Fe.sub.3 O.sub.4, CrO.sub.2 and iron alloys are used for this purpose and the coercive forces obtainable with iron oxides, cobalt-modified iron oxides and CrO.sub.2 under technically practicable marginal conditions have their upper limit at about 80 kA m.sup.-1. A marked increase in the coercive forces can only be obtained with particles of iron alloys.
Extremely finely divided iron alloys with high coercive forces are used for this purpose; the profile of their requirements may be found in DE-A 3 226 937, FIG. 3.
According to DE-A 2 014 500, these powders may be prepared by isomorphic reduction of suitable starting compounds. This procedure, however, entails considerable difficulties. Thus when goethite is used as starting material, complete destruction of the desired needle shape may occur so that only insufficient coercive forces are obtained. Further, severe sintering is observed due to the fine state of subdivision, and this again results in inadequate products. Another problem lies in the pyrophoricity of the reduced alloy powders, which must be eliminated by additional measures such as surface oxidation of the particles, after-treatment or combinations of these procedures. This step is also necessary for suppressing the damage to the properties of the resulting magnetic recording carriers by corrosion.
Various attempts have been made to solve this problem. In DE-A 2 553 635, for example, the goethite compound used as starting material is covered with tin oxide and titanium oxide and products having an average needle length of from 0.3 to 1 .mu.m are obtained. According to DE-A 2 212 934, the precursor compound is coated with Co ions and/or Ni ions. Lastly, EP-A 056 257 discloses a process in which the precursor compound is treated with silicon dioxide and aluminium oxide.
The best values for coercivity are obtained from precursor compounds doped with nickel but these pigments are not ecologically harmless.
It is an object of the present invention to provide suitable alloy powders which are free from the above described disadvantages.